You currently have javascript disabled. Several functions may not work. Please re-enable javascript to access full functionality.

Welcome to Ultimate Subaru Message Board, my lurker friend!

Welcome to Ultimate Subaru Message Board, an unparalleled Subaru community full of the greatest Subaru gurus and modders on the planet! We offer technical information and discussion about all things Subaru, the best and most popular all wheel drive vehicles ever created.

We offer all this information for free to everyone, even lurkers like you! All we ask in return is that you sign up and give back some of what you get out - without our awesome registered users none of this would be possible! Plus, you get way more great stuff as a member! Lurk to lose, participate to WIN*!

Say hello and join the conversation

Subscribe to topics and forums to get automatic updates

Get your own profile and make new friends

Classifieds with all sorts of Subaru goodies

Photo hosting in our gallery

Meet other cool people with cool cars

Seriously, what are you waiting for? Make your life more fulfilling and join today! You and your Subaru won't regret it, we guarantee** it.

* The joy of participation and being generally awesome constitutes winning** Not an actual guarantee, but seriously, you probably won't regret it!

CEL 2000 legacy

2000 legacy wagon 97K CEL came on today i thougth oh boy another knock sensor well went immediately to Auto Zone the reading they got was, Catalyst below threshold bank #1, i figured O2 sensor, anyone else with this problem ever? btw I unhooked the battery and cleared the code, made sure all the spark plug wires were seated properly, then hooked the battery back up and took it for a short drive without the CEL coming back on, if anyone has any input id greatly appreciate it.

Basically this means that the oxygen sensor downstream (after) the catalytic converter on bank 1 is detecting that the converter is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

Symptoms

You will likely not notice any drivability problems, although there may be symptoms.

Causes

A code P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for

An oxygen sensor is not reading (functioning) properly

The engine coolant temperature sensor is not working properly

Damaged exhaust manifold / catalytic converter / exhaust pipe

Retarded spark timing

Possible Solutions

The simplest thing to do is to reset the code and see if it comes back.

Next step is to measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter). The voltage should be low and steady (~200mV, depends on the vehicle). If the voltage varies (e.g. 100mV to 600mV) then it's time to replace the catalytic converter. In fact, it would be a good idea to test each oxygen O2 sensor while you're at it.

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.

P0420 OBD Trouble Code

Technical Description

Catalyst System Efficiency Below Threshold (Bank 1)

What does that mean?

Basically this means that the oxygen sensor downstream (after) the catalytic converter on bank 1 is detecting that the converter is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

Symptoms

You will likely not notice any drivability problems, although there may be symptoms.

Causes

A code P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for

An oxygen sensor is not reading (functioning) properly

The engine coolant temperature sensor is not working properly

Damaged exhaust manifold / catalytic converter / exhaust pipe

Retarded spark timing

Possible Solutions

The simplest thing to do is to reset the code and see if it comes back.

Next step is to measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter). The voltage should be low and steady (~200mV, depends on the vehicle). If the voltage varies (e.g. 100mV to 600mV) then it's time to replace the catalytic converter. In fact, it would be a good idea to test each oxygen O2 sensor while you're at it.

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.

wow did that ever answer my question thanks loads, that is what makes this such a great site, i did clear the code and after a few short trips it has not come back so i will keep my fingers crossed, all that 02 sensor and catylitic converter talk is scary.